The present invention relates to the field of processing optical signals.
Many optical image processing operations such as edge enhancement may be performed by suitable linear spatial filters. A broader class of operations is made available by extending this technique to nonlinear filtering. An example of this is the use of logarithmic nonlinearities for signal compression or conversion of multiplicative to additive noise. In accordance with the present invention, we demonstrate the use of phase preserving thresholding (with the linear transmission of high intensities) in the Fourier plane for reducing additive signal dependent noise, such as coherent artifact noise, image defects and spatial light modulator nonuniformity.
Additive signal dependent noise is much more difficult to reduce than independent additive noise. Coherent artifact noise, uniformity noise, defect noise and film grain noise may produce important limitations in the operation of coherent optical processing devices. It has been shown that each of these noise sources contributes similarly in the Fourier transform plane by adding a signal dependent noise component to the signal transform. If this added noise transform term is weak relative to the signal transform, a nonlinear optical thresholding mechanism operating in the Fourier plane will separate the relatively intense signal peaks by attenuating the noise between the signal peaks. This noise reduction often works with input amplitudes which have a small signal relative to noise. We used the thresholding mechanism of photorefractive two-beam coupling to reduce strong artifact or defect noise in plane wave illumination of a bar chart transparency. There is no noticeable reduction in the resolution of the signal used in our measurements. Furthermore, no clean signal pattern is used as a reference signal in this noise reduction technique.